Lesson
Content
Most of this weeks background information is available in Chapter
5. Below is a summary of some additional information.
Turbulence
There are four causes of turbulence
1. Convection
2. Surface Roughness
3. Gravity Waves
4. Wind Shear
Convection: In convection, hot air rises freely
becuase it is warmer than the environment. This is sometimes referred
to as thermal turbulence. There are two basic types of atmospheric
convection: forced and free. In forced convection the air is mechanically
pushed, such as along fronts or mountains. In free convection, air
moves vertically without any external help. Forced convection involves
neutral or stable air. Free convection requires unstable air. Stable
air reduces convection and thus has less thermal turbulence.
Surface Roughness: When a plane lands on a windy
day we experience increased turbulence during the descent. These
eddies are caused by surface friction or the roughness of the surface.
The degree of turbulence depends on the wind speed, stability, and
the roughness of the surface. This type of turbulence is also referred
to as mechanical turbulence.
Atmospheric Gravity Waves: Periodic motions can
result when stable air is moved upward or downward. Since the vertical
motion is in the vertical, gravity plays a major role in restoring
the displaced air to is original location. Theiwave-like oscillations
move away from the original point of disturbance and are known as
atmospheric gravity waves. They often occur in clear skies and so
are difficult to see.
Wind Shear: Wind shear is a gradient in the wind
speed and/or wind direction over some distance. Horizontal wind shear
is a change in wind over a horizontal distance. Vertical wind shear
is a change in wind over a vertical distance. Vertical wind shear
occurs near the surface due to friction and in the vicinity of certain
stable layers. Horizontal wind shear near the ground is caused by
the wind interacting with the surface. Away from the surface, horizontal
wind shear is caused by fronts, jet streams and strong convection.
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Classification
of Turbulence
There are four classifications of turbulence:
1. Light- Unsecured objects will stay at rest. A half-filled cup of coffee
won't spill.
2. Moderate- Unsecured objects move about; passengers will wake-up if they
are asleep, and a half-filled cup of coffee will spill.
3. Severe - Passengers who aren't buckled in fumble around and try to buckle-up
as they are bounced about avoiding your coffee cup which is bouncing around.
4. Extreme - Control of aircraft might be impossible, let alone a control over
a cup which once contained coffee.
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Rising
parcels
Remember, initially a parcel has the same temperature
as air around it. When we raise the parcel the air pressure is lower;
molecules
push
on "sides" to expand parcel - this is work; less energy
(kinetic energy) cooler temperature. Rising parcels of air always
cool due to expansion. Sinking or subsiding air always warms by compression.
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